In high speed applications such as analog to digital conversion in image processing systems, it is often necessary to produce a current from a voltage such that the current is precisely proportional to the voltage despite drift due to time and temperature and distortion due to switching currents interfering with the switched current. These problems are especially serious when the switching times are in the picosecond range to provide integration conversions in the nanosecond range.
A further problem occurs when the integration window must be capable of being varied over a wide range, for example from 6 to 48 nanoseconds: at shorter durations, less charge is accumulated and so a lower voltage is applied to the A/D converter so that the whole input range of the A/D converter is not fully utilized.
To overcome the above described problems the present inventor developed a voltage to current conversion switching system which is described in U.S. Pat. No. 5,041,794 (the '794 patent). That patent is incorporated herein in its entirety by reference. In that patent the high speed switching in the voltage to current conversion system necessary in video applications is accomplished by high speed transistors. However, by implementing the high speed switching in this manner the system's gain stability over its operating temperature range is severely affected. To improve the gain stability, operational amplifiers are used in place of the discrete transistors as is shown in FIG. 5 of that patent. However, this embodiment is useful only at lower speeds in the 50 nanosecond range and is not useful for video applications. Moreover, in that embodiment two pairs of operational amplifiers (two stages) must be used; a first pair of operational amplifiers, one in each channel, are used to offset the input voltage signal and a second pair of operational amplifiers, one in each channel, are used in conjunction with transistors to accomplish the voltage to current conversion.